Critical Error Jira 6.1.4

I tried to do a "JIRA restore data from a backup" in test server (from 5.2.5 to old version) through Jira interface (System > Import Export)

But it didn't worked, here the error message that i received:com.atlassian.crowd.exception.runtime.UserNotFoundException: User <P998416> does not exist

I couldn't do anything...I preferred to return to the new version (from 5.2.5 to 6.1.4)So I stopped pid of jira and I replaced the workspace-jira (containing folders caches,data,export...) and deployment directory (webapps) of 5.2.5 release to 6.1.4 release(I used to proceed thus,because the upgrade 5.2.5 to 6.1.4 has always worked )